1. Field of the Invention
The present invention relates generally to a bearing for a motor. More particularly, this invention relates to a resin injected bearing, for use in a small motor for a power window or a wiper of a vehicle.
2. Description of the Related Art
In general, the mechanism for driving a power window is equipped with a small motor which is combined with a worm reduction gear having a worm gear mechanism. The housing of the worm reduction gear is attached to the casing of the motor. The rotor of the motor, the worm screw, etc. are housed in the space defined by the housing and the casing. The worm screw is coupled to the distal end portion of the rotor shaft.
The rotor integrated with the worm screw, is supported by three radial bearings provided at the distal end portion of the worm screw, the rear end portion of the rotor and the middle between the rotor and worm screw. To avoid adverse influence caused by misalignment of the axes of the casing and the housing, when they are assembled together, the conventional drive mechanism for a power window provides a large clearance (about 0.1 mm), between the distal end portion of the worm shaft and the associated radial bearing.
If the clearance between the distal end portion of the worm shaft and the associated radial bearing is large, when the motor is rotated, two events may occur. The first is that the worm shaft rotates while contacting the radial bearing, and the second is that the worm shaft rotates without contacting the radial bearing. In this case, the noise generated by the driving motor will vary between those two events, and may be offensive or unpleasant to the ear.
The power window should normally be locked at the uppermost position and the lowermost position. With a large clearance provided between the distal end portion of the worm shaft and the associated radial bearing, the worm screw may escape from the worm wheel at the time the window glass is locked, thus resulting in insufficient engagement therebetween. As a result, a large load is applied only to the middle of the three radial bearings, thus interfering with the smooth activation of the rotor. In addition, if the centering of the housing of the worm reduction gear is improper with respect to the motor casing, bending stress will be applied to the rotor shaft such that the motor cannot provide the original driving power.
As an attempted solution to the aforementioned problems, the work precision of members, such as the housing and the motor casing, may be improved to make the clearance between the distal end portion of the worm shaft and the associated radial bearing smaller than the conventional clearance. The use of the state-of-the-art technology to provide such a high working precision will undesirably result in significant rise in the production cost of motors.
Japanese Patent Examined Publication No. 61-56701 discloses a motor which has a thrust bearing formed of synthetic resin injected into the motor casing. As the distal end of the worm coupled to the rotor abuts on this thrust bearing, the movement of the worm screw and rotor in the thrust direction is restricted, thus ensuring their position.
When the distal end portion of the worm shaft directly contacts the thrust bearing which is formed of synthetic resin, the resin will be worn out through long usage, thus disabling the continuous and accurate positioning of the worm screw and rotor. Further, bearings formed by resin injection have an inherent shortcoming that cooling-originated contraction will form a clearance between itself and the inner wall of the motor casing or housing, to thereby impair the sealability of the inner space.